23,082 research outputs found
Coherent structures in fully-developed pipe turbulence
A turbulent mean profile for pipe flow is prescribed which closely matches
experimental observations. The nature of perturbations superimposed upon this
profile is then considered. Optimal growth calculations predict two distinct
classes of structures, clearly associated with near-wall and large-scale
structures. Quantitative correspondence of the spanwise wavelength of
wall-structures with experimental observations is very good. The response to
harmonic forcing is also considered, and the linear growth tested with direct
numerical simulation of forced turbulence. Despite the very simple eddy
viscosity assumption, this linear approach predicts well the surprisingly large
growth of outer-scale modes in the bulk flow.
Un profil moyen turbulent est prescrit dans une conduite cylindrique, en
adequation avec les observations experimentales. Nous considerons ensuite la
nature des perturbations a cet ecoulement synthetique. Le calcul des
croissances optimales predit deux types de structures, associees respectivement
aux structures de proche-paroi et de grande echelle. Un excellent accord
quantitatif est trouve avec les resultats experimentaux quant a la longueur
d'onde transversale. La reponse harmonique est egalement etudiee, et la
croissance lineaire observee comparee a des simulations numeriques directes de
turbulence forcee. Malgre de l'hypothese simple de type `Eddy viscosity', cette
approche lineaire predit efficacement la croissance spectaculaire des modes de
grande echelle au coeur de l'ecoulement.Comment: 5 pages; Congres Francais de Mecanique, Marseille (2009
To enhance collaborative learning and practice network knowledge with a virtualization laboratory and online synchronous discussion
This work is licensed under a Creative Commons Attribution 4.0 Internatinal License.Recently, various computer networking courses have included additional laboratory classes in order to enhance students' learning achievement. However, these classes need to establish a suitable laboratory where each student can connect network devices to configure and test functions within different network topologies. In this case, the Linux operating system can be used to operate network devices and the virtualization technique can include multiple OSs for supporting a significant number of students. In previous research, the virtualization application was successfully applied in a laboratory, but focused only on individual assignments. The present study extends previous research by designing the Networking Virtualization-Based Laboratory (NVBLab), which requires collaborative learning among the experimental students. The students were divided into an experimental group and a control group for the experiment. The experimental group performed their laboratory assignments using NVBLab, whereas the control group completed them on virtual machines (VMs) that were installed on their personal computers. Moreover, students using NVBLab were provided with an online synchronous discussion (OSD) feature that enabled them to communicate with others. The laboratory assignments were divided into two parts: Basic Labs and Advanced Labs. The results show that the experimental group significantly outperformed the control group in two Advanced Labs and the post-test after Advanced Labs. Furthermore, the experimental group's activities were better than those of the control group based on the total average of the command count per laboratory. Finally, the findings of the interviews and questionnaires with the experimental group reveal that NVBLab was helpful during and after laboratory class
Form Factors Calculated on the Light-Front
A consistent treatment of decay is given on the
light-front. The to transition form factors are calculated in the
entire physical range of momentum transfer for the first time. The
valence-quark contribution is obtained using relativistic light-front wave
functions. Higher quark-antiquark Fock-state of the -meson bound state is
represented effectively by the configuration, and its effect
is calculated in the chiral perturbation theory. Wave function renormalization
is taken into account consistently. The contribution dominates
near the zero-recoil point ( GeV), and decreases rapidly as
the recoil momentum increases. We find that the calculated form factor
follows approximately a dipole -dependence in the entire range
of momentum transfer.Comment: Revtex, 19 pages, 9 figure
Dramatic Mobility Enhancements in Doped SrTiO3 Thin Films by Defect Management
We report bulk-quality n-type SrTiO3 (n-SrTiO3) thin films fabricated by
pulsed laser deposition, with electron mobility as high as 6600 cm2 V-1 s-1 at
2 K and carrier density as low as 2.0 x 10^18cm-3 (~ 0.02 at. %), far exceeding
previous pulsed laser deposition films. This result stems from precise
strontium and oxygen vacancy defect chemistry management, providing a general
approach for defect control in complex oxide heteroepitaxy.Comment: 13 pages, 4 figure
Inverter-Based Low-Voltage CCII- Design and Its Filter Application
This paper presents a negative type second-generation current conveyor (CCII-). It is based on an inverter-based low-voltage error amplifier, and a negative current mirror. The CCII- could be operated in a very low supply voltage such as ±0.5V. The proposed CCII- has wide input voltage range (±0.24V), wide output voltage (±0.24V) and wide output current range (±24mA). The proposed CCII- has no on-chip capacitors, so it can be designed with standard CMOS digital processes. Moreover, the architecture of the proposed circuit without cascoded MOSFET transistors is easily designed and suitable for low-voltage operation. The proposed CCII- has been fabricated in TSMC 0.18μm CMOS processes and it occupies 1189.91 x 1178.43μm2 (include PADs). It can also be validated by low voltage CCII filters
Stoichiometry control of the electronic properties of the LaAlO_3/SrTiO_3 heterointerface
We investigate the effect of the laser parameters of pulsed laser deposition
on the film stoichiometry and electronic properties of LaAlO_3/SrTiO_3 (001)
heterostructures. The La/Al ratio in the LaAlO_3 films was varied over a wide
range from 0.88 to 1.15, and was found to have a strong effect on the interface
conductivity. In particular, the carrier density is modulated over more than
two orders of magnitude. The film lattice expansion, caused by cation
vacancies, is found to be the important functional parameter. These results can
be understood to arise from the variations in the electrostatic boundary
conditions, and their resolution, with stoichiometry.Comment: 4 pages, 3 figures, submitted for publicatio
Fermi surface and superconductivity in low-density high-mobility {\delta}-doped SrTiO3
The electronic structure of low-density n-type SrTiO3 delta-doped
heterostructures is investigated by angular dependent Shubnikov-de Haas
oscillations. In addition to a controllable crossover from a three- to
two-dimensional Fermi surface, clear beating patterns for decreasing dopant
layer thicknesses are found. These indicate the lifting of the degeneracy of
the conduction band due to subband quantization in the two-dimensional limit.
Analysis of the temperature-dependent oscillations shows that similar effective
masses are found for all components, associated with the splitting of the light
electron pocket. The dimensionality crossover in the superconducting state is
found to be distinct from the normal state, resulting in a rich phase diagram
as a function of dopant layer thickness.Comment: 4 pages, 5 figures, submitted for publicatio
Negative energy and stability in scalar-tensor gravity
Linearized gravitational waves in Brans-Dicke and scalar-tensor theories
carry negative energy. A gauge-invariant analysis shows that the background
Minkowski space is stable at the classical level with respect to linear scalar
and tensor inhomogeneous perturbations.Comment: 9 pages, latex, to appear in Phys. Rev.
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